Well she's a loner, she gets mildly hostile with anyone who tries to break her shell, she once killed a guy by giving him such a scathing Internet retort that he burst into flames... and she enjoys the company of mice
I will point out that in no Persona game to memory has every Arcana been held by a playable character. Many of the cards have been given to supporting characters such as NPCs with close ties to the story, NPCs with close ties to NPCs with close ties to the story, or just interesting shop keeps.